Hydrostatic machine

ABSTRACT

A hydrostatic machine, e.g. a hydrostatic pump or hydrostatic motor operating in a closed fluid circuit and having a housing into which leakage of the hydraulic medium can occur, is provided with check valves communicating between the housing and the main ports to enable leakage fluid to pass directly into the main hydraulic circuit.

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o m C m numb Aa me fiLW Rm a n D m Y w m HmA P M 7 PrimaryExaminer-Edgar W. Geoghegan Attorney-Karl F. Ross Appl. No.: 79,532

[22] Filed:

[57] ABSTRACT A hydrostatic machine, e.g. a hydrostatic pump or [30]Foreign Application Priority Data Oct. 10, 1969 Germany.............P 1951 234.6 hydrostatic motor p i g a closed fluid circuit and having ahousing into which leakage of the hydraulic medium can occur, isprovided with check [52] US. Cl. ..............60/53 A, 60/52 US,60/DIG. 5,

60/51 51 Int. Cl. 39/02, Fl6h 39/10 f mmumcatmg heme? d [58] Field ofSearch.....60/53 A, 52 US, DIG. 5, 53 R mam P0rts to enable leakage mudto P duectly the main hydraulic circuit.

9 Claim, 5 Drawing Figures References Cited UNITED STATES PATENTS2,597,050 5/1952 Audemar...................60/51UX EP'A'TENTEDms' 1 m2sum 1 0F 4 Franz Forsfer 1 VENTOR FIG.

PATE'NTEDAIIB- I 1912 3.680.312

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A (Ross Attorney 1 HYDROSTATIC MACHINE FIELD OF THE INVENTION My presentinvention relates to hydraulic machines and, more particularly, tohydrostatic machines operating in a closed fluid circuit.

BACKGROUND OF THE INVENTION Hydrostatic machines, e.g. hydrostatic pumpsand motors, generally are provided with a closed circuit and a pair ofmain ports connected with a hydraulic load and serving as discharge orintake ports, depending upon the sense of operation of the machine.Among the important hydrostatic machines of this class are the socalledaxial-piston pumps and axial-piston motors which serve, respectively, todisplace the hydrostatic fluid under the drive of a power source, e.g.an intemalcombustion engine or an electric motor and/or may be driven byhydraulic fluid pressure to operate, in turn, a mechanical member suchas the output shaft. Axialpiston devices and the principles involvedtherein are described in FLUID POWER, US. Government Printing Office,1966, at pages 109-112 and pages 199 and following, respectively.

For the most part, an axial-piston pump may comprise a rotary cylinderbarrel provided with a plurality of angularly equispaced cylinder boressuccessively communicating with a pair of kidney-shapedfluid-distribution apertures on a fluid-distribution surface againstwhich the cylinder barrel is held under axial pressure. The pistonswithin the cylinders are reciprocated by virtue of rotation of thebarrel and the fact that the pistons bear upon a control surface whichis inclined to the axes of rotation of the barrel so that during abouthalf of each rotation the pistons are urged inwardly while the pistonsare able to move outwardly during the remainder of each rotation.

Inward displacement of the pistons, of course, corresponds to areduction in the volume of the chamber behind the piston to expel fluidin the form of a hydraulic medium through one kidney-shaped aperturewhile outward movement of the piston expands the chamber to draw thehydraulic medium into the cylinder bore from the other kidney-shapedaperture. The kidneyshaped apertures, of course, are connected to thedischarge and intake ports of the hydraulic machine, respectively,depending upon the angle and direction of tilt of the control surface,the displacement of the pump and the function of the main port (ashigh-pressure or low-pressure port) can be established. The pump shaftmay be connected to the control surface to drive the latter and may alsobe coupled with the barrel via means as described, for example, in thecommonly assigned copending application Ser. No. 68,254 filed Aug. 31,70 by Walter HEYL. A hydrostatic motor operating in accordance with thesame principles, will generally comprise a cylinder barrel having asurface perpendicular to its axis of rotation abuting afluid-distribution surface whose arcuate apertures communicate with theindividual cylinder bores opening at this surface. The pistons may bearagainst an inclined control surface and are coupled with an output shaftvia the latter so that fluid entering through one of the aperturesforces the piston successively outwardly to drive the barrel and,consequently, rotate the shaft. Inclination of the control surface inthis case, determines the speed of the shaft and the torque delivered toany load which may be coupled therewith.

It is not uncommon to interconnect the discharge port of the pump withthe intake side of the motor and the outlet side of the motor with theintake side of the pump by suitable conduits and thereby create ahydrostatic drive or transmission in which the transmission ratiobetween the input shaft of the pump and the output shaft of the motor isestablished by the inclination of the barrel axes of the hydrostaticmachines to the axis of the respective shaft. Such transmissions may bewholly contained in a common housing or may be mounted remote from oneanother so that they can be connected by relatively longlines. Oneadvantage of the hydrostatic transmission is precisely the possibilityof providing the hydrostatic pump in the vicinity of a prime mover orother energy source, while the hydrostatic motors are mounted directlyadjacent the load driven thereby. Transmissions of this nature have beenfound to be particularly suitable in vehicular applications wherein theprime mover is an internal-combustion engine and the load is the vehiclewheel. Of course, a number of hydrostatic motors can be connected to asingle pump or a number of pumps may be provided to service a singlehydrostatic motor. In general, however, it is found to be advantageousto provide each hydrostatic pump with a hydrostatic motor in a closedfluid path.

It should be understood. that the term closed fluid path is intendedherein to refer to a system in which the pump is connected directly tothe load, i.e. each of the conduits communicating with the ports of thepump run to the corresponding ports of the motor. While one of theconduits may be operated as a high-pressure transmission line while theother conduit is at low pressure, the hydrostatic machines are generallyreversible to interchange the functions of these lines. Furthermore,both machines may be provided in a closed housing so that the entiretransmission network and fluid supply is contained within this housingfrom which only the input and output shafts emerge. In closedcirculating paths of the character described, it is necessary to holdthe low-pressure side at a predetermined pressure level and an auxiliarypump and/or a pressure-regulating valve may be used to this end.

It will be appreciated that effective operation of the axial-piston pumpor axial-piston motor is accompanied and in fact may require someleakage of the hydraulic medium from the system, e. g. at thefluid-distribution surfaces of a valve plate and the rotating cylinderdrum, respectively. When a closed circulating path was required, it hasbeen necessary in some priorart devices to provide a further conduitbetween the pump and load to convey the leakage fluid from onehydrostatic machine to the other. This is, of course, a significantdisadvantage when the pump is greatly removed from the motor and whencomplex mechanisms are interposed between them. In an excavatingmachine, for example, the hydrostatic pump may be mounted upon a chassiscarrying a turntable which, in turn, is provided with the excavatingscoops and like devices. When the turntable is used, it rotatesrelatively to the chassis. Hence hydrostatic motors carried by theturntable must be connected through a rotating seal with the hydrostaticpump and the need for an additional conduit between the hydrostaticmotor and the hydrostatic pump complicates such systems to a furtherextent. It will be appreciated that conduits for the indicated purposesmust be capable of withstanding pressure and the use of an additionalconduit involves increasing complexities because of the associatedseals.

In open hydraulic circuits, moreover, i.e. those using a collectingreservoir into which fluid flows in a pressureless state, it is commonto connect the leakagefluid conduit to the reservoir so that the fluidreturns to the power cycle, the leakage path being pressureless. It isalso known to provide, in such systems (open fluidcirculating path),pumps which are disposed'directly in the reservoir and leakage can occurdirectly into the reservoir. These systems make use of a relativelylarge housing for the pump and possible pressurization of the fluidtherein to prevent cavitation at the inlet side of the pump. In allcases, however, techniques which have been found to be satisfactory forhandling the leakage fluid of the open hydraulic circuit, i.e. thoseusing fluid reservoirs, have not been found to be practical orsatisfactory in closed fluid circuits while the need for addedleakage-fluid conduits renders earlier closed systems expensive andcomplex.

OBJECTS OF THE INVENTION It is the principal object of the presentinvention to provide an improved hydrostatic machine arrangement of theclosed-circulation type wherein the aforementioneddisadvantages areobviated.

Another object of my invention is to provide an improvedhydrostatic'machine, e.g. axial-piston pump or axial-piston motor, inwhich the handling of leakage fluids is simplified, which is of lowercost and simpler operation than earlier systems, and which can be usedfor arrangements such as excavators in which two hydraulic machines areinterconnected but separated by means creating sealing difficulties.

SUMMARY OF THE INVENTION The above and other objects of the invention,which will become apparent hereinafter, are attained in a hydraulicmachine which comprises a housing, preferably closely surrounding theaxial-piston barrel which is connected directly with the low-pressureport of the machine, i.e. the intake port of a hydrostatic pump or thedischarge port of a hydrostatic motor, via a check valve designed topermit unidirectional flow of the hydraulic medium from the housingsurrounding the barrel into the low-pressure port. The housing sealinglycloses the hydrostatic machine and, when the ports of the machine arefunctionally interchangeable, both of them may be connected with thehousing chamber by respective check valves, only the check valvecommunicating with the low-pressure port being open to permit thepressure differential thereacross to feed the leakage fluid into thelow-pressure side. The

high-pressure side, of course, maintains its check valve low-pressureside of the hydrostatic machine, a check valve connecting the interiorof the housing with the low-pressure duct, conduit or port, and ahydrostatic machine disposed within this housing and having a leakagepath opening from the machine into the housing whereby the leakagemedium can traverse the check valve into the low-pressure ducts. Aseparate leakage-fluid collector is thereby eliminated, the system doesnot require a separate line between the pump and motor for conductingthe leakage fluid and compensating for leakage losses, and only twoducts need bridge the hydrostatic pump and motor. Furthermore, thecooling of the hydrostatic machine is improved by virtue of the factthat there is a continuous circulation of the leakage medium from themachine into the space surrounding the machine and enclosed by thehousing, and from the housing into the low-pressure side of the machine.

The present invention is applicable to hydrostatic pumps and motors, theintake of the fonner being the low-pressure side whereas the dischargeof the latter is at low pressure. However invention can also apply tomore than one hydrostatic machine in a single housing, i.e. adouble-pump assembly in which two hydrostatic pumps are provided withina single housing, two hydrostatic drives including a pump and one ormore motors, the housing in each case being generally closed. However,it has beenfound that increasing the size of the housing to accommodatemore than one hydrostatic machine opens the door to difficulties withrespect to maintaining the seal of the housing and it is thereforepreferred to provide a housing for each machine which closely surroundsthe axial-piston drum thereof. It should be noted that one not onlyachieves a saving in the cost of a leakage-fluid duct when the presentinvention is used, but also reduces the complexities of the pump andmotor structures themselves since fittings, ports, chambers and likestructure associated with the leakage ducts are eliminated as well. Itis possible by such simplification of the overall structure to eliminatethe tendency of the machine to leak and thereby reduce maintenance andsurveillance.

The reduction of the leakage losses from the main closed hydrauliccircuit into the housing surrounding the hydrostatic machine is aconsequence in part of the high back pressure maintained in the sealedhousing and, therefore, prevalent at the outlet side of the leakagepath. As a result, no feed pump need be used to compensate for leakageloss in a great many cases and, wherever a feed ptunp is required in asystem under the present invention, it may be dimensioned to have asmaller capacity and energy consumption than the feed pumps which havebeen used heretofore with similar hydraulic machines operating, forexample, in closed circuits with pressurelesshousings or the like.

When no feed pump is required, I have found it to be advantageous toprovide, at one or more locations along the closed hydraulic path,equalization reservoirs or hydropneumatic accumulators adapted todeliver,

while maintaining the pressure in the housing, fluid to the latter tocompensate for the leakage losses from the main hydraulic circuit. Suchaccumulators may be of the type described at pages 86 89 of FLUID POWER.cited earlier. The accumulator, which is maintained at the predeterminedpressure within the sealed housing,

compensates for changes in the volume of fluid available in themaincirculating path as a result of thermal expansion and contraction of thefluid and the conduits containingv same, elastic yieldability of theconduit walls, etc.

When ahydropneumatic accumulator is employed, I have found itadvantageous after a period determined by experience, e.g. a thousandoperating hours, to charge the accumulator and restore the pressuretherein. Of course, the accumulator should only be provided. at thelow-pressure sideof the, closed circulation pathand, when the. hydraulicmachine is reversible so that the ports alternate in function betweenhigh-pressure. andilow-pressure ports, reversing-valve means is used inaccordance. with the invention to connect the low-pressure side, withthe accumulator at all times. Two such accumulators may, of course, beprovided and connectedwith thesides of the hydraulic network by cutoffvalve means so that the accumulator currently at the high-pressure. sideis blocked .while the other accumulator is rendered effective.Furthermore, when a pair of interconnected hydrostatic machines areemployed, the. accumulator can communicate directly with the interior'ofone of the machine housings or the common housing of both machines tomaintain the pressure in the low-pressure side of the networksubstantially constant-indirectly.

When a feed pump is provided, i.e. when the hydrostatic machine isprovided with a pump designed to deliver the-hydraulic medium to theclosed fluid path to compensatefor leakage losses, a secondarycirculation is established between the housing chamber of one machinepreferably the motor and the low-pressure line thereof. A cooling systemcan be provided in the latter case at the discharge side of the feedpump. The cooler may be a conventional radiator built onto the housingof the apparatus or other conventional heat-dissipating device. Hencethe interior of the housing is constantly rinsed with fresh coolhydraulic medium and the machine is able to operate with increasingefficiency.

Accordingto still another feature of this invention, the hydrostaticmachine forms part of a hydraulic transmission driven by aninternal-combustion engine and the working fluid of the hydrostaticdrive is also the lubricant for the. internal combustion engine. Thefeed pump and cooling pump can thereby constitute the means formaintaining the predetermined pressure within the housing, the means forcooling the latter, and the means for lubricating the engine.

DESCRIPTION OF THE DRAWING The above and other objects, features andadvantages -of the present invention will become more readily apparentfrom the following description, reference being made to the accompanyingdrawing in which:

FIG. 1 is an axial cross-sectional view through an embodiment of ahydrostatic machine according to the invention; FIG. 2 is aside-elevational view, partly in axial section and partly indiagrammatic form, illustrating another embodiment of the inventionwherein the hydrostatic machine employs a feed pump;

FIG. 3 is a view similar to FIG. 2 of a hydrostatic transmissionaccording to the invention;

FIG. 4 is a view similar to FIG. 1 illustrating how the invention isapplied to a double-pump assembly; and

FIG. 5 is an elevational view diagrammatically illustrating theapplication of the invention to another hydrostatic transmission havinga single housing.

SPECIFIC DESCRIPTION In FIG. 1 I have shown a hydrostatic machine havinga shaft 1 which may serve as the input shaft of the hydrostatic pump andwhich is joumaled in a pair of axially spaced bearings 5 and 6 of ahousing 3, 4 interconnected at 3a. The shaft 1 carries a cylinder drum 2provided with a plurality of angularly equispaced axially extendingcylinder bores 7, each slidably receiving a piston 8 shiftable into andout of the cylinder bore 7 parallel to its own axis and the axis of theshaft 1. Each of the pistons 8 is provided with a swivel head or ball 8awhich is received swivelably within a shoe plate 9 of annularconfiguration, slidably bearing against a control disk 10 whose surface10a is inclined at the angle a to the axis A of the shaft. Spring means(not shown) may be provided to ensure that the pistons 8 and the shoe 9seats firmly against the control surface 10a at all operating speeds ofthe rotary machine.

The housing 3, 4 comprises a bell-shaped member 3 having a neck 3b inwhich the bearing 5 is received and which is formed with a lip-type sealcapable of maintaining a subatmospheric pressure in a chamber 16surrounding the cylinder barrel 2. The latter is provided with apertures7a which open at a valve surface 2a bearing against the opposing surfaceof a valve or distributing plate 15 composed of low-friction material,e.g. bronze, seated against the plate 4. The plate 15 is provided with apair of arcuate apertures 15a and 15b, respectively registering withseveral of the apertures 7 a as the barrel 2 rotates relative to theplate 15. Such arcuate apertures are disclosed in the aforementionedpublication. Each of the apertures 15a and 15b registers, in turn, witha respective connecting passage 13 or 14 communicating, in turn, withthe ports 11 and 12 to which suitable conduit means may be connected forjoining the hydrostatic machine of FIG. 1 in a closed hydraulic circuit.A typical circuit is that illustrated in FIG. 3. When port 11 forms thehigh-pressure side of the machine, port 12 represents the low-pressureside and vice versa. The pressure at the low-pressure side may bemaintained constant by a feed pump or pressure-regulation valveinterconnecting the high and lowpressure sides. The low-pressure sidepreferably is held at about 6 to 8 atmospheres gauge. The same pressureis maintained by seal 23 within the housing. When the system is ahydrostatic motor, the discharge is at this pressure while, when themachine is a hydrostatic pump, the supply or intake is at this pressure.

The interior 16 of the housing communicates via respective bores 17 and18, formed directly in plate 4, with respective check valves 20 andcompartments l9 and 21 operating, respectively, into the passages 13 and14 mentioned earlier. The check valves 20 are poled, oriented andconstructed to permit unidirectional flow of fluid from the housing intothe low-pressure conduit or branch when a pressure differential in favorof such flow is established and to block reverse flow of fluid under anycircumstances. Since reverse flow is blocked when the pressuredifl'erentialfavors an outflow from the network into the chamber 16,whenever the operat- I ing high pressure is maintained in one of thenetworks, the corresponding check valve blocks flow therethrough andonly the other check valve can operate, this only when the pressurewithin the housing exceeds the pressure within the low-pressure side asindicated.

The shaft 1 may be coupled to a prime mover, e. g. an electric motor oran internal combustion engine for use of the machine as a pump, or maybe connected to a load such as the driving wheels of an automotivevehicle having a hydrostatic transmission. In the mode of operation ofthe machine as a pump, the barrel 2 is rotatably entrained by the shaft1 while the pistons 8 ride with the shoe v9 along the control surface10a which is inclined to the axis of rotation of the barrel as notedearlier. As the pistons 8 are shifted between their fully extendedposition and a fully retracted position, they vary the size of thechamber 7 behind the piston and thereby draw fluid through one port andforce it out through .the other in a repetitive intake/discharge cycle.When the machine is operated as a motor, hydraulic fluid is delivered bya hydrostatic pump at one port to drive the pistons outwardly as thebarrel swings into registry with that port, the fluid then passing atlow pressure into the discharge side for return to the pump. In eithercase, one of the ports 11 or 12 will be a high-pressure port while theother is the low-pressure port.

When port 11 is under high pressure and port 12 is under low pressure,the fluid in line 13 is likewise at an elevated pressure to bias thecheck valve 19 into a closed position. Fluid from the chamber 16 cannotenter the closed hydraulic network via line ,17. However, the normalpressure in the housing chamber 16 is 6 to 8 atmospheres (gauge) andsufflces, when thepressure drops at the low-pressure duct 14 and port12, to bleed the leakage fluid from the chamber 16 into the hydraulicline 14 for return to the main circulating path. The fluid within thechamber 16 in part derives from leakage at the control surfaces 2a. Whenthe control surface 10a is adjusted to vary the functions of the portsor the sense of rotation is altered, only the check valve associatedwith the low-pressure side will be operative. The high-pressure checkvalve will invariably be closed.

In FIG. 2, I show a hydrostatic machine according to the invention whichis constituted as a pump and embodies many of the features alreadydescribed in connection with FIG. 1. In this embodiment, the shaft 101,extending out of the, housing portion 103 is connected with a drivemeans such as an inlet combustion engine or electric motor. The shaft10l is provided with an extension. 25 beyond the bearing 106 running toa feed pump 26, the latter being bolted onto the housing portion 104forming the fluid-distribution ports 111 and 112 as well as ducts 113and 114 as previously described.

The intake line 27 of the pump 26 is fed from a reservoir 28 while thepressure side of the pump26 is connected via line 29 to the usingcompartment 116 through the wall 103 thereof, preferably at the bottomof the wall. A return line 31 communicates with the compartment 1 16 atthe upper side thereof via a fitting which is provided with a checkvalve 32 allowing unidirectional flowing from the housing to thereservoir 28. Valve 32 is provided with a strong spring 32a so that itsimultaneously constitutes a pressure-regulating valve maintaining anadjustable pressure in the compartment 1 16 at about 6 to 8 atmospheres.In thiscase, the reservoir 28 can be open to the atmosphere and canconstitute a heat exchanger or heat-dissipating cooler directly. As analternative, the reservoir 28 may be closed to maintain a given pressurewithin the system and the cooler may be provided as a separate heatexchanger or radiator in a fluid circuit with the pump 1 26. The pump26, consequently, circulates fresh fluid through the interior of thehousing constantly with the advantages already set forth. In general,the machine of FIG. 2 operates in the manner previously described inFIG. 1 when the barrel 102 is rotated by shaft 101 to shift the pistons107 in the cylinder bores 108 of the closed hydraulic circuit and permitthe check valve or 122 associated with the low-pressure side to connectthe respective passage 113 or 114 with the interior of .the housing.

The hydrostatic drive illustrated in FIG. 3 comprise in accordance withthe usual practice, a hydrostatic pump whose shaft 201 is connected withan internal combustion engine and whose hydrostatic motor has its shaft301 connected with a load. The shafts 201 and 301 are rotatablyconnected with the cylinder barrels 202, 302 whose pistons 208, 308 areaxially shiftable, via inclined control surfaces not shown, within thecylinder bores 207, 307 to displace hydrostatic fluid along a closedpath inter-connecting the, ports 211 and 311 via a line 33 and the ports212, 312 via a line 34. It will be appreciated that the lines 33 and 34may be relatively long when the pump and the motor are to be separatedby some distance or can be eliminated when a single support block isprovided in place of the separate fluid distribution plates 204, 304.The

remainder of the system, including the fluid-distribution antifrictionplates 215, 315 the ports 217 and 317 connected to the low-pressure sideof the network and the check valves 220, 320 and222, 322 is, of course,identical with the corresponding parts of the system of FIG. 1. Inaddition, a pressure-equalization reservoir 36, in which a compressedgas is maintained in the compartment 36a to form a yieldable cushion forthe membrane 36b connected with line 35 and the chamber 216. A valve 360serves to permit recharging of the accumulator. With expansion of thefluid within the transmission as a result of heating, there is a volumeincrease which is taken up by the accumulator 36 with compression of thegas cushion therein. Should there be a leak from the housing or anelastic yielding of the ducts from a housing and portions thereof,additional fluid is delivered by the accumulator. No separate feed pumpis necessary.

In FIG. 4, I show a system wherein a housing 403, 404 is common to apair of cylinder drums 402, with respective shafts 401 driven via gears401a from a gear 40lb on the crankshaft 4010 of an automotive vehicle orlike installation using a double pump aggregate. Since separate closedhydraulic networks may be provided with the low-pressure ports 412 atthe same pressure and the high-pressure ports 411 at respective elevatedpressures, a check valve 420 is provided to connect both ports 412 withthe interior 416 of the BEST AVAILABLE COPY housing. The other checkvalves 422 for the high-pressure side are provided as previouslydescribed.

FIG. shows a system wherein the cylinder drums 502 and 602 of thehydrostatic pump and hydrostatic motor are mounted in a common housing503 and have input and output shafts 501 and 601 respectively connectedto a source of rotary movement and a load. The control member 510 ishere shown to be pivotal via lever 510' to adjust the displacement ofthe pump and a similar means may be provided for tilting the controlplate 610. A single valve block 504 is here used with the intake side ofthe duct 514 connected with the check valve 520. Pressure within thesystem may be controlled by a regulating valve 650 connecting the highand low pressure lines or by a feed pump 526 driven by gearing 561 fromthe pump shaft 501. The major distinction between the system of FIG. 5and that of FIG. 4 is the single housing for the barrels of both thepump and motor of this latter system.

In FIG. 5, I also show a distributing valve 520a by which the checkvalve 520 and the duct associated pressure corresponding substantiallyto that of said low-pressure passage; and a duct wholly within saidhousing constituting the sole leakage-fluid path therefrom andconnecting said low-pressure passage with said chamber, and a checkvalve in said duct for unidirectional flow of fluid between said chamberand said low-pressure passage and blocking reverse flow of therewith maybe switched between the current lowpressure line 514 and thehigh-pressure line 513 when the hydraulic machines are reversed asdescribed earlier. in this case only a single check valve need be used.The valve 520a may be coupled as represented by the dot-dash line 52%with lever 510 to effect automatically the change-over.

I claim:

1. A hydraulic system comprising a hydrostatic pump and a hydrostaticmotor constituting hydrostatic machines; means connecting said machinesin a closed fluid-circulating path and including a high-pressure passageand a low-pressure passage; a pressure-retentive housing surrounding atleast one of said machines and-forming a chamber maintainable at anelevated fluid through said duct between said low-pressure passage andsaid chamber.

2. The hydraulic system defined in. claim 1 wherein a respective ductconnects each of said passages with said chamber and is provided with arespective check valvepermitting unidirectional flow of fluid from thechamber into said passages.

3. The hydraulic system defined in claim 1, further comprisingdistributing valve means for selectively connecting said duct with oneof said passages.

4. The hydraulic system defined in claim 1, further comprising a feedpump connected with said chamber for circulating fluid therethrough.

5. The hydraulic system defined in claim 4, further comprising aheat-dissipating cooler in a hydraulic circuit with said feed pump.

6. The hydraulic system defined in claim 1 wherein both said machinesare provided within said housing.

s l l h in fi "fl 1% hyil i ulic system defined in claim 1, furthercomprising means for maintaining the pressure within said chamber at alevel of substantially 6 to 8 atmospheres gauge.

9. The hydraulic system defined in claim 1 wherein said housing enclosesa pair of hydrostatic pumps, coupled together in a double-pumpaggregate.

1. A hydraulic system comprising a hydrostatic pump and a hydrostaticmotor constituting hydrostatic machines; means connecting said machinesin a closed fluid-circulating path and including a high-pressure passageand a low-pressure passage; a pressure-retentive housing surrounding atleast one of said machines and forming a chamber maintainable at anelevated pressure corresponding substantially to that of saidlow-pressure passage; and a duct wholly within said housing constitutingthe sole leakage-fluid path therefrom and connecting said lowpressurepassage with said chamber, and a check valve in said duct forunidirectional flow of fluid between said chamber and said low-pressurepassage and blocking reverse flow of fluid through said duct betweensaid low-pressure passage and said chamber.
 2. The hydraulic systemdefined in claim 1 wherein a respective duct connects each of saidpassages with said chamber and is provided with a respective check valvepermitting unidirectional flow of fluid from the chamber into saidpassages.
 3. The hydraulic system defined in claim 1, further comprisingdistributing valve means for selectively connecting said duct with oneof said passages.
 4. The hydraulic system defined in claim 1, furthercomprising a feed pump connected with said chamber for circulating fluidtherethrough.
 5. The hydraulic system defined in claim 4, furthercomprising a heat-dissipating cooler in a hydraulic circuit with saidfeed pump.
 6. The hydraulic system defined in claim 1 wherein both saidmachines are provided within said housing.
 7. The hydraulic systemdefined in claim 1, further comprising a hydropneumatic accumulatorconnected to said chamber for maintaining a predetermined pressure leveltherein.
 8. The hydraulic system defined in claim 1, further comprisingmeans for maintaining the pressure within said chamber at a level ofsubstantially 6 to 8 atmospheres gauge.
 9. The hydraulic system definedin claim 1 wherein said housing encloses a pair of hydrostatic pumps,coupled together in a double-pump aggregate.